This invention relates to a polyurethane-based adhesive, which is particularly well suited for bonding shoe soles to shoe uppers in footwear manufacturing.
Modern shoe industry employs machines capable of producing large quantities of the necessary shoe parts. Those parts must be joined together to form the final product. In industrial manufacturing processes, joining of shoe parts is often accomplished by adhesive bonding. This step must produce adequate strength bonds in a short time to avoid slowing down the production lines. The most critical bonding operation is the attachment of the shoe uppers to the sole. The sole can be made of a variety of natural or synthetic plastic or elastomeric materials, such as polyvinyl chloride, thermoplastic rubber, styrene/butadiene copolymers, and thermoset polyurethanes.
The adhesives currently most widely used are solvent solutions of synthetic elastomers, such as polychloroprene or polyurethane. The bonding process involves coating the substrates with an adhesive solution, evaporating the solvent, then "reactivating" the adhesive film by the application of mild heat before assembly. Activation temperatures above about 70.degree. C must be avoided because they cause distortion of many heat-sensitive shoe materials.
It would be advantageous to replace the solvent solutions with solvent-free adhesives. Suitable systems are known and are used in other bonding operations. They comprise fluid isocyanate-terminated prepolymers that can be chain-extended or "cured" by admixture with diamine curing agents such as methlenedianiline. The combination of curing agent and an isocyanate-terminated prepolymer should have adequate potlife after mixing but sufficiently high reaction rate for continuous line applications. Unmodified methylenedianiline, when used with typical urethane prepolymers, gives adhesives having too short a potlife to be applicable in sole attaching operations. A complex of methylenedianiline with sodium chloride in a mole ratio of 3:1 is a known curing agent for urethane prepolymers. A typical dispersion of this complex is available from E. I. Du Pont de Nemours & Co, Wilmington, Delaware, under the name "Caytur 21". Mixtures of "Caytur 21" with urethane prepolymers have a very long potlife but are slow to cure unless activated at temperatures above 120.degree. C, that would destroy many typical shoe materials. The activation temperature of methylenedianiline/sodium chloride complexes can be reduced to about 90.degree. C by addition of certain active hydrogen compounds including urea, according to the teachings of U.S. Pat. No. 3,891,606, but this is still too high a temperature for most shoe materials. Therefore, a polyurethane-based adhesive capable of producing in a short time and at a moderate temperature bonds having good strength is greatly needed.